1. Field of the Invention
The invention relates to a method for measuring concentration of a gas component based on two measurement methods of direct absorption spectroscopy and wavelength modulation spectroscopy, where the wavelength of the light of a tunable light source is varied periodically over an absorption line of interest for the gas component and, in the case of wavelength modulation spectroscopy, the wavelength of the light is additionally sinusoidally modulated at a high frequency and with a small amplitude, and the intensity of the light is detected after transradiation of the measurement gas and processed to yield a measurement result.
2. Description of the Related Art
DE 102 38 356 A1 discloses a method in which two measurement methods are applied alternately in consecutive periods.
Laser spectrometers are used, in particular, for optical gas analysis in process measurement technology. In this case, a laser diode generates light in the infrared region that is guided through the process gas (measurement gas) to be measured and subsequently detected. The wavelength of the light is tuned to a specific absorption line of the gas component respectively to be measured, the laser diode periodically scanning the absorption line as a function of wavelength. For this purpose, the laser diode is driven periodically by a ramp-shaped or triangular current signal. During the direct absorption spectroscopy (DAS), the concentration of the gas component of interest is determined directly from the reduction in light intensity (absorption) detected at the position of the absorption line. In the case of the wavelength modulation spectroscopy (WMS), during the comparatively slow wavelength-dependent scanning of the absorption line, the wavelength of the generated light is additionally sinusoidally modulated at a high frequency and with small amplitude. The profile of the absorption line is not linear. As a result, harmonics of higher order are also generated in the detector signal or measurement signal. The measurement signal is usually evaluated for a higher harmonic, preferably the second harmonic by phase-sensitive lock-in technique (WMS-If). For small modulation parameters, the detection of the nth harmonic is directly proportional to the nth derivative of the direct measurement signal, and so the designation of derivative absorption spectroscopy is also used for the WMS-If. In the case of the WMS, the evaluation of the measurement signal requires an analytical approximation description of the absorption line.
Wavelength modulation spectroscopy and absorption spectroscopy have specific advantages and disadvantages. WMS is advantageous, in particular, for measuring of low concentrations, because it is better at filtering out noise from the measurement signal. However, in the case of higher concentrations, the approximations required to evaluate the measurement signal become increasingly inaccurate, and so the measuring error rises. The situation is reversed in the case of DAS; because of the higher noise sensitivity, the measuring error is higher for small concentrations. However, since there is no need for an approximation description of the absorption line, the measuring accuracy is better with increasing concentration because the useful signal becomes stronger. It is only at very high concentrations (saturation of the absorption) that the measurement method again becomes more inaccurate.
In a method known from U.S. Pat. No. 7,616,316 B2, there is a switch between various operating modes, in particular between both high and WMS at low concentrations of the gas component to be measured. Thus, it is respectively the measurement method that seems to be most suitable that is applied.
In the case of the method known from the above named DE 102 38 356 A1, the two measurement methods WMS and DAS are applied alternately in consecutive periods, i.e., quasi-simultaneously, and the detected signals are likewise fed alternately to two separate averagings and evaluated in parallel. In the case of WMS evaluation, the results of the DAS evaluation can be used, for example, for calibration. This delivers the freedom from calibration of the direct absorption spectroscopy, and the accuracy of the wavelength modulation spectroscopy.